Process for making silver powder particles with very small size crystallites

ABSTRACT

The process for making silver powder particles with very small size crystallites uses a combination of gum arabic and maleic acid with the reduction of a silver salt with ascorbic acid. Silver thick film paste containing these silver powder particles can be used in electronic applications to form electrodes for semiconductor devices and, in particular, solar cells.

FIELD OF THE INVENTION

The invention is directed to a process for making silver powderparticles with very small size crystallites. These silver powderparticles are particularly useful in electronic applications.

BACKGROUND OF THE INVENTION

Silver powder is used in the electronics industry for the manufacture ofconductor thick film pastes. The thick film pastes are screen printedonto substrates forming conductive circuit patterns. These circuits arethen dried and fired to volatilize the liquid organic vehicle and sinterthe silver particles.

Many processes currently used to manufacture metal powders can beapplied to the production of silver powders. For example, thermaldecomposition processes, electrochemical processes, physical processessuch as atomization or milling and chemical reduction processes can beused. Thermal decomposition processes tend to produce powders that arespongy, agglomerated, and very porous whereas electrochemical processesproduce powders that are crystalline in shape and very large. Physicalprocesses are generally used to make flaked materials or very largespherical particles. Chemical precipitation processes produce silverpowders with a range of sizes and shapes.

Silver powders used in electronic applications are generallymanufactured using chemical precipitation processes. Silver powder isproduced by chemical reduction in which an aqueous solution of a solublesalt of silver is reacted with an appropriate reducing agent underconditions such that silver powder can be precipitated. Inorganicreducing agents including hydrazine, sulfite salts and formate salts canproduce powders which are very coarse in size, are irregularly shapedand have a large particle size distribution due to aggregation. Organicreducing agents such as alcohols, sugars or aldehydes are used withalkali hydroxides to reduce silver nitrate. The reduction reaction isvery fast; hard to control and produces a powder contaminated withresidual alkali ions. Although small in size (<1 μm), these powders tendto have an irregular shape with a wide distribution of particle sizesthat do not pack well. It is difficult to control the sintering of thesetypes of silver powders and they do not provide adequate line resolutionin thick film conductor circuits.

Therefore, there is a need for a process to produce silver powderspherical particles comprising very small size crystallites and having ad₅₀ particle size in the range of 0.5 to 3.5 μm to provide silver thickfilm paste with improved sintering properties.

SUMMARY OF THE INVENTION

This invention provides a process for making silver powder particles,the process comprising:

-   -   (a) preparing an acidic aqueous silver salt solution comprising        a water soluble silver salt dissolved in deionized water;    -   (b) preparing an acidic aqueous reducing and particle modifier        solution comprising:        -   (i) a reducing agent selected from the group consisting of            ascorbic acid, ascorbates and mixtures;        -   (ii) nitric acid;        -   (iii) maleic acid; and        -   (iv) deionized water, wherein the pH of the acidic aqueous            reducing and particle modifier solution is adjusted by the            addition of a base to between 2.5 and 6;    -   (c) maintaining the acidic aqueous silver salt solution and the        acidic reducing and surface morphology modifier solution at the        same temperature, wherein the temperature is in the range of        10° C. to 65° C., while stirring each the solution during and        after preparation;    -   (d) adding the acidic aqueous silver salt solution to the acidic        aqueous reducing and particle modifier solution while stirring        to make a reaction mixture and maintaining the temperature of        the reaction mixture at the temperature of (c), wherein the        silver powder particles precipitate and are contained within the        resulting final aqueous solution; and    -   (e) increasing the temperature of the final aqueous solution to        a temperature in the range of 65° C. to 80° C. while continuing        stirring;        one or both of the acidic aqueous silver salt solution and the        acidic aqueous reducing and particle modifier solution further        comprising gum arabic and the silver powder particles comprising        crystallites of size less than or equal to 32 nm as determined        by X-ray diffraction and the Scherrer formula; and the silver        powder particles having a d₅₀ in the range of 0.5 to 3.5 μm.

Also provided is the above process further comprising:

-   -   (f) separating the silver powder particles from the final        aqueous solution;    -   (g) washing the silver powder particles with deionized water;        and    -   (h) drying the silver powder particles.

Also provided are the silver powder particles made by the above process,silver thick film paste made with the silver powder particles and asemiconductor device comprising an electrode that prior to firingcomprises the silver thick film paste.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a scanning electron microscope image at a magnification of10,000 of the silver powder particles made in the Example. The d₅₀particle size is 1.6 μm.

FIG. 2 is a scanning electron microscope image at a magnification of10,000 of the silver powder particles made in the ComparativeExperiment. The d₅₀ particle size is 8.7 μm.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a process for making spherically-shaped silverpowder particles with very small size crystallites. A combination of gumarabic and maleic acid is used with the reduction of a silver salt withascorbic acid to control the crystallite size. The process is areductive process in which silver particles comprising crystallites lessthan 30 nm in size as determined by X-ray diffraction and the Scherrerformula are precipitated by adding together an acidic aqueous solutionof a water soluble silver salt and an acidic aqueous reducing andparticle modifier solution containing a reducing agent, nitric acid, gumarabic, and maleic acid. In addition, the pH of the acidic aqueousreducing and particle modifier solution is adjusted to between 2.5 and 6by the addition of a base, preferably NaOH. In one group of embodimentsthe pH of the acidic aqueous reducing and particle modifier solution isadjusted to between 3 and 5 and in another group of embodiments the pHis adjusted to 4.

The acidic aqueous silver salt solution is prepared by adding a watersoluble silver salt to deionized water. Any water soluble silver salt,e.g., silver nitrate, silver phosphate, and silver sulfate, can be used.Silver nitrate is preferred. No complexing agents are used which couldprovide side reactions that affect the reduction and type of particlesproduced. Nitric acid can be added to increase the acidity.

The process can be run at concentrations up to 1.2 moles of silver perliter of final aqueous solution. It is preferred to run the process atconcentrations of 0.47 to 0.8 moles of silver per liter of final aqueoussolution. These relatively high concentrations of silver make themanufacturing process cost effective.

The acidic aqueous reducing and particle modifier solution is preparedby adding the various components, i.e., a reducing agent, maleic acidand nitric acid, to deionized water. Suitable reducing agents for theprocess are ascorbic acids such L-ascorbic acid and D-ascorbic acid andrelated ascorbates such as sodium ascorbate. Ascorbic acid is preferred.

One or both of the acidic aqueous silver salt solution and the acidicaqueous reducing and particle modifier solution further contain gumarabic.

In one embodiment, the acidic aqueous reducing and particle modifiersolution also contains a metal colloid selected from the groupconsisting of gold colloid and silver colloid. A suitable metal colloidis gold colloid or silver colloid. Gold colloid is preferred.

The components can be added to the deionized water in various orders. Inone embodiment, gum arabic is the first component added to the deionizedwater whether it is in the preparation of the acidic aqueous silver saltsolution or the acidic aqueous reducing and particle modifier solution.The gum arabic is added to the deionized water and then stirred for aperiod of time before adding the other components. In the Examples, forthe acidic aqueous silver salt solution 10 g of gum arabic was added to125 g or 250 g of deionized water and for the acidic aqueous reducingand particle modifier solution 14 g of gum arabic was added to 375 g ofdeionized water and the mixture stirred for an hour before adding anyother component. Times for stirring smaller or larger quantities couldbe adjusted accordingly. In this embodiment nitric acid acid, maleicacid, ascorbic acid and gold colloid solution are added in that order toform the acidic aqueous reducing and particle modifier solution.Stirring for a time as indicated above should occur following additionof the gum arabic no matter the order of adding the components.

The order of preparing the acidic aqueous silver salt solution and theacidic reducing and particle modifier solution is no important. Theacidic aqueous silver salt solution can be prepared before, after, orcontemporaneously with the acidic reducing and particle modifiersolution. The acidic aqueous silver salt solution is added to the acidicreducing and particle modifier solution. The addition is carried outslowly. For example, with the quantities of solutions used in theExamples, i.e., 125 and 250 g of deionized water for the acidic aqueoussilver salt solution and 375 and 750 g of deionized water for the acidicaqueous reducing and particle modifier solution, the acidic aqueoussilver salt solution was added to the acidic reducing and particlemodifier solution over a period of one hour. Smaller quantities could bemixed over a shorter time interval and larger quantities over a longertime period. The reaction mixture is stirred during the addition.

In this process the acidic aqueous silver salt solution and the acidicreducing and particle modifier solution are both maintained at the sametemperature, i.e., a temperature in the range of 10° C. to 65° C. andeach solution is stirred until they are mixed. In some embodiments thesolutions are maintained at a temperature in the range of 10° C. to 35°C. In one such embodiment the solutions are maintained at 25° C. Whenthe two solutions are mixed to form the reaction mixture, the reactionmixture is maintained at that same temperature.

The mixture of the acidic aqueous silver salt solution and the acidicreducing and particle modifier solution results in the precipitation ofthe silver particles that are contained within the resulting finalaqueous solution. Following precipitation, the temperature of the finalaqueous solution is heated to a temperature in the range of 65° C. to80° C. In some embodiments the temperature is in the range of 65° C. to75° C. In one such embodiment the temperature is 70° C. The finalaqueous solution containing the silver particles is maintained at thistemperature for about an hour and is stirred during that time.

The silver particles are then separated from the final aqueous solutionby filtration or other suitable liquid-solid separation operation andthe solids are washed with deionized water until the conductivity of thewash water is 100 microSiemans or less. The silver particles are thendried.

The silver powder particles made by the process of the invention have aparticular set of physical characteristics. The silver particles aredescribed herein as spherically-shaped. It can be seen from the scanningelectron microscope (SEM) image of FIG. 1 that the particles aregenerally spherical in shape but are not perfect spheres. The averageparticle size determined from the SEM image is in the range of 1 to 2.5μm. The silver powder particles are comprised of crystallites less thanor equal to 32 nm in size as determined by X-ray diffraction and theScherrer formula. In one group of embodiments the crystallites are lessthan or equal to 30 nm in size. The silver powder particles have a d₅₀in the range of 0.5 to 3.5 μm and a size distribution such that(d₉₀−d₁₀)/d₅₀<2.2. The quantity (d₉₀−d₁₀)/d₅₀ is a measure of the sizedistribution of the particles. Since (d₉₀−d₁₀)/d₅₀<2.2, it indicates thenarrow size distribution of the particles, i.e., the uniformity ofparticle size and the correspondly highly dispersible nature of thesilver powders comprising these particles. The silver particles have asolids content of in excess of 99% as determined by thermogravimetricanalysis. Thermomechanical analysis (TMA) is used herein as an indicatorof sinterability. A pellet was formed from the silver particles andsubjected to a temperature regime. The observed per cent shrinkage,i.e., the dimensional change of the pellet, is a measure of thesinterability of the particles. The results are presented herein as TMAequal to the percent dimensional change, a negative per cent changesince it represents a shrinkage. The silver particles of the inventionshowed TMA dimensional changes, i.e., shrinkages, of at least −10%.

The silver powder particles made by this invention can be used in thickfilm paste applications, including thick films for front sidemetallization of photovoltaic solar cells and thick films for othersemiconductor devices.

This invention also provides the silver powder particles made by theprocess of this invention. These silver powder particles can be used inthick film paste. The structure of these silver particles will lend themto be more readily sintered and provide improved thick film conductors.Also provided is a semiconductor device, e.g., a solar cell, comprisingan electrode that prior to firing comprises the silver thick film paste.

Silver Thick Film Paste

This invention also provides a silver thick film paste comprised of thesilver powder particles made by the process of this invention and glassfrit dispersed in an organic medium. As used herein, “thick film paste”refers to a composition which after being deposited on a substrate andfired has a thickness of 1 to 100 μm.

The glass frit compositions are described herein as includingpercentages of certain components. The percentages are the percentagesof the components used in the starling material that was subsequentlyprocessed as described herein to form a glass composition. Thecomposition contains certain components and the percentages of thosecomponents are expressed as a percentage of the corresponding oxide orfluoride form. The weight percentages of the glass frit components arebased on the total weight of the glass composition. A certain portion ofvolatile species may be released during the process of making the glass.An example of a volatile species is oxygen.

Various glass frit compositions are useful in the silver thick filmpastes of the invention. The glass frit used has a softening point of300 to 600° C. The glass frit compositions described herein are notlimiting. Minor substitutions of additional ingredients can be madewithout substantially changing the desired properties of the glasscomposition. For example, substitutions of glass formers such as 0-3 wt% P₂O₅, 0-3 wt % GeO₂ and 0-3 wt % V₂O₅ can be used either individuallyor in combination to achieve similar performance.

The glass frit compositions can also contain one or morefluorine-containing components such as salts of fluorine, fluorides andmetal oxyfluoride compounds. Such fluorine-containing componentsinclude, but are not limited to BiF₃, AlF₃, NaF, LiF, KF, CsF, PbF₂,ZrF₄, TiF₄ and ZnF₂.

Exemplary lead free glass compositions contain one or more of SiO₂,B₂O₃, Al₂O₃, Bi₂O₃, BiF_(3,) ZnO, ZrO_(2,) CuO, Na₂O, NaF, Li₂O, LiF,K₂O, and KF. In various embodiments the compositions comprise thefollowing oxide constituents in the compositional ranges, the SiO₂ is 17to 26 wt %, 19 to 24 wt %, or 20 to 22 wt %; the B₂O₃ is 2 to 9 wt %, 3to 7 wt %; or 3 to 4 wt %; the Al₂O₃ is 0.1 to 5 wt %, 0.2 to 2.5 wt %,or 0.2 to 0.3 wt %; the Bi₂O₃ is 0 to 65 wt %, 25 to 64 wt %, or 46 to64 wt %; the BiF₃ is 0 to 67 wt %, 0 to 43 wt %, or 0 to 19 wt %; theZrO₂ is 0 to 5 wt %, 2 to 5 wt %, or 4 to 5 wt %; the TiO₂ is 1 to 7 wt%, 1 to 5 wt %, or 1 to 3 wt %; CuO is 0 to 3 wt % or 2 to 3 wt %; Na₂Ois 0 to 2 wt % or 1 to 2 wt %; NaF is 0 to 3 wt % or 2 to 3 wt %; Li₂Ois 0 to 2 wt % or 1 to 2 wt %; and LiF is 0 to 3 wt % or 2 to 3 wt %.Some or all of the Na₂O or Li₂O can be replaced with K₂O and some or allof the NaF or LiF can be replaced with KF to create a glass withproperties similar to the compositions listed above.

In other embodiments, the glass frit compositions can include one ormore of a third set of components: CeO₂, SnO₂, Ga₂O₃, In₂O₃, NiO, MoO₃,WO₃, Y₂O₃, La₂O₃, Nd₂O₃, FeO, HfO₂, Cr₂O₃, CdO, Nb₂O₅, Ag₂O, Sb₂O₃, andmetal halides (e.g. NaCl, KBr, NaI).

Exemplary lead containing glass compositions comprise the followingoxide constituents in the compositional range of 0-36 wt % SiO₂, 0-9 wt% Al₂O₃, 0-19 wt % B₂O₃, 16-84 wt % PbO, 0-4 wt % CuO, 0-24 wt % ZnO,0-52 wt % Bi₂O₃, 0-8 wt % ZrO₂, 0-20 wt % TiO₂, 0-5 wt % P₂O₅, and 3-34wt % PbF₂. In other embodiments relating to glasses containing bismuthoxide, the glass frit composition contains 4-26 wt % SiO₂, 0-1 wt %Al₂O₃, 0-8 wt % B₂O₃, 20-52 wt % PbO, 0-4 wt % ZnO, 6-52 wt % Bi₂O₃, 2-7wt % TiO₂, 5-29 wt % PbF₂, 0-1 wt % Na₂O and 0-1 wt % Li₂O. In stillother embodiments relating to glasses containing 15-25 wt % ZnO, theglass frit comprises 5-36 wt % SiO₂, 0-9 wt % Al₂O₃, 0-19 wt % B₂O₃,17-64 wt % PbO, 0-39 wt % Bi₂O₃, 0-6 wt % TiO₂, 0-5 wt % P₂O₅ and 6-29wt % PbF₂. In various of these embodiments containing ZnO, the glassfrit compositions comprises 5-15 wt % SiO₂ and/or 20-29 wt % PbF₂ and/or0-3 wt % ZrO₂ or 0.1-2.5 wt % ZrO₂. Embodiments containing copper oxideand/or alkali modifiers comprise 25-35 wt % SiO₂, 0-4 wt % Al₂O₃, 3-19wt % B₂O₃, 17-52 wt % PbO, 0-12 wt % ZnO, 0-7 wt % Bi₂O₃, 0-5 wt % TiO₂,7-22 wt % PbF₂, 0-3 wt % CuO, 0-4 wt % Na₂O and 0-1 wt % Li₂O.

An exemplary method for producing the glass frits described herein is byconventional glass making techniques. Ingredients are weighed then mixedin the desired proportions and heated in a furnace to form a melt inplatinum alloy crucibles or other suitable metal or ceramic crucibles.As indicated above, oxides as well as fluoride or oxyfluoride salts canbe used as raw materials. Alternatively, salts, such as nitrate,nitrites, carbonate, or hydrates, which decompose into oxide, fluorides,or oxyfluorides at temperature below the glass melting temperature canbe used as raw materials. Heating is conducted to a peak temperature oftypically 800-1400° C. and for a time such that the melt becomesentirely liquid, homogeneous, and free of any residual decompositionproducts of the raw materials. The molten glass is then quenched betweencounter rotating stainless steel rollers to form a 10-15 mil thickplatelet of glass. The resulting glass platelet was then milled to forma glass frit powder with its 50% volume distribution set between to adesired target (e.g. 0.8-1.5 μm). Alternative synthesis techniques suchas water quenching, sol-gel, spray pyrolysis, or others appropriate formaking powder forms of glass can be employed.

The organic medium used in the silver hick film paste is a solution of apolymer in a solvent. The organic medium can also contain thickeners,stabilizers, surfactants and/or other common additives. In oneembodiment, the polymer is ethyl cellulose. Other exemplary polymersinclude ethylhydroxyethyl cellulose, wood rosin, mixtures of ethylcellulose and phenolic resins, polymethacrylates of lower alcohols, andmonobutyl ether of ethylene glycol monoacetate, or mixtures thereof. Thesolvents useful in the organic medium of the silver thick film pastecompositions include ester alcohols and terpenes such as alpha- orbeta-terpineol or mixtures thereof with other solvents such as kerosene,dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexyleneglycol and high boiling alcohols and alcohol esters. The organic mediumcan also contain volatile liquids for promoting rapid hardening afterapplication on the substrate.

The thick film silver composition is adjusted to a predetermined,screen-printable viscosity with the organic medium.

The inorganic components, i.e., the silver powder particles and theglass frit are typically mixed with the organic medium by mechanicalmixing to form a viscous paste composition.

The ratio of organic medium in the silver thick film paste compositionto the inorganic components in the dispersion is dependent on the methodof applying the paste and the kind of organic medium used, and it canvary. The dispersion will typically contain 70 to 95 wt % of inorganiccomponents and 5 to 30 wt % of organic medium in order to obtain goodwetting. The weight percents (wt %) used herein are based on the totalweight of the silver thick film paste composition. Typically, thepolymer present in the organic medium is in the range of 8 wt % to 11 wt% of the weight of the total composition.

In one embodiment, the silver thick film paste contains 65 to 90 wt %silver powder, 0.1 to 8 wt % glass frit and 5 to 30 wt % organic medium.In another embodiment the silver thick film paste contains 70 to 85 wt %silver powder, 1 to 6 wt % glass frit and 10 to 25 wt % organic medium.In still another embodiment the silver thick film paste contains 78 to83 wt % silver powder, 2 to 5 wt % glass frit and 13 to 20 wt % organicmedium.

Semiconductor Device; Solar Cell

The invention also provides a method of making a semiconductor device,e.g., a solar cell or a photodiode. The semiconductor device has anelectrode, e.g., a front side electrode of a solar cell or a photodiode,wherein prior to firing the electrode is comprised of a silver thickfilm paste of the invention.

The method of manufacturing a semiconductor device, comprises the stepsof:

-   -   (a) providing a semiconductor substrate, one or more insulating        films, and the silver thick film paste of the invention;    -   (b) applying the insulating film to the semiconductor substrate,    -   (c) applying the silver thick film paste to the insulating film        on the semiconductor substrate, and    -   (d) firing the semiconductor substrate, the insulating film and        the silver thick film paste composition.

Exemplary semiconductor substrates useful in the methods and devicesdescribed herein include, but are not limited to, single-crystalsilicon, multicrystalline silicon, and ribbon silicon. The semiconductorsubstrate may be doped with phosphorus and boron to form a p/n junction.

The semiconductor substrates can vary in size (length×width) andthickness. As an example, the thickness of the semiconductor substrateis 50 to 500 μm; 100 to 300 μm; or 140 to 200 μm. The length and widthof the semiconductor substrate are each 100 to 250 mm; 125 to 200 mm; or125 to 156 mm.

Typically an anti-reflection coating is formed on the front side of asolar cell. Exemplary anti-refection coating materials useful in themethods and devices described herein include, but are not limited to:silicon nitride, silicon oxide, titanium oxide, SiN_(x):H, hydrogenatedamorphous silicon nitride, and silicon oxide/titanium oxide film. Thecoating can be formed by plasma enhanced chemical vapor deposition(PECVD), CVD, and/or other known techniques known. In an embodiment inwhich the coating is silicon nitride, the silicon nitride film can beformed by PECVD, thermal CVD, or physical vapor deposition (PVD). In anembodiment in which the insulating film is silicon oxide, the siliconoxide film can be formed by thermal oxidation, thermal CVD, plasma CVD,or PVD.

The silver thick film paste of the invention can be applied to theanti-reflective coated semiconductor substrate by a variety of methodssuch as screen-printing, ink-jet printing, coextrusion, syringedispensing, direct writing, and aerosol ink jet printing. The paste canbe applied in a pattern and in a predetermined shape and at apredetermined position. In one embodiment, the paste is used to formboth the conductive fingers and busbars of the front-side electrode. Insuch an embodiment, the width of the lines of the conductive fingers are20 to 200 μm, 40 to 150 μm, or 60 to 100 μm and the thickness of thelines of the conductive fingers are 5 to 50 μm, 10 to 35 μm, or 15 to 30μm.

The paste coated on the ARC-coated semiconductor substrate can be dried,for example, for 0.5 to 10 minutes during which time the volatilesolvents and organics of the organic medium are removed.

The dried paste is fired by heating to a maximum temperature of between500 and 940° C. for a duration of 1 second to 2 minutes. In oneembodiment, the maximum silicon wafer temperature reached during firingranges from 650 to 80° C. for a duration of 1 to 10 seconds. In afurther embodiment, the electrode formed from the silver thick filmpaste is fired in an atmosphere composed of a mixed gas of oxygen andnitrogen. In another embodiment, the electrode formed from theconductive thick film paste is fired above the organic medium removaltemperature in an inert atmosphere not containing oxygen. This firingprocess removes any remaining organic medium and sinters the glass fritwith the silver powder and any metal oxide present to form an electrode.Typically, the burnout and firing is carried out in a belt furnace. Thetemperature range in the burnout zone, during which time the remainingorganic medium is removed, is between 500 and 700° C. The temperature inthe firing zone is between 860 and 940° C.

During firing, the fired electrode, preferably the fingers, reacts withand penetrates the anti-reflective coating, thereby making electricalcontact with the silicon substrate.

In a further embodiment, prior to firing, other conductive and deviceenhancing materials are applied to the back side of the semiconductordevice and cofired or sequentially fired with the paste compositions ofthe invention. The materials serve as electrical contacts, passivatinglayers, and solderable tabbing areas.

In one embodiment, the back side conductive material contains aluminumor aluminum and silver.

In a still further embodiment the materials applied to the opposite typeregion of the device are adjacent to the materials described herein dueto the p and n region being formed side by side. Such devices place allmetal contact materials on the non illuminated back side of the deviceto maximize incident light on the illuminated front side.

Measurements

The particle size distribution numbers (d₁₀, d₅₀, d₉₀) used herein arebased on a volume (mass) distribution. The particle sizes were measuredusing a Microtrac® Particle Size Analyzer from Leeds and Northrup. Thed₁₀, d₅₀ and d₉₀ are the equivalent diameters that represent the 10thpercentile, the median or 50th percentile and the 90th percentile of theparticle sizes, respectively, as measured by volume. That is, the d₅₀(d₁₀, d₉₀) is a value on the distribution such that 50 volume % (10volume %, 90 volume %) of the particles have an equivalent diameter ofthis value or less.

The solids content of the silver particles was determined bythermogravimetric analysis, i.e., by a weight loss method after heatingat 850° C. for 10 minutes.

The size of the crystallites making up the silver powder particles wasdetermined by X-ray diffraction and the Scherrer formula. The X-raydiffractometer used was a Rigaku Rint RAD-rb. The Cu target provided awavelength of 0.15405 nm. The Bragg plane was the (111). In the Scherrerformula:

L (111)=0.94λ/(β cos θ),

L is the crystallite size, λ is the wavelength, β is the line broadeningat half the peak maximum intensity (full width−half maximum) in radiansand θ is the Bragg angle.

The TMA was carried out as follows. A pellet was formed by pouring0.9-1.1 g of silver powder into a die and onto a cylindrical metalinsert serving as the bottom. A second cylindrical metal is insertedinto the die on top of the silver powder so that the powder issandwiched between the cylindrical metal inserts. The top insert is thenfirmly tapped 6 times with a mallet. The pellet is placed in a BargalTMA:Q400 analyzer which measured the dimensional change as a function oftemperature. The temperature was raised from room temperature at a rateof 10° C./min to 650° C. and the pellet is maintained at thattemperature for 10 minutes. The final per cent dimensional change isreported as the TMA for that powder pellet. A negative value indicatesshrinkage and a positive value expansion of the pellet.

EXAMPLES

The following Examples and discussion are offered to further illustrate,but not limit the process of this invention.

Example 1

The acidic aqueous silver salt solution was prepared by adding 10 g ofgum arabic to 250 g of deionized water. This solution was kept at 25° C.while continuously stirring for an hour. 80 g of silver nitrate was thenadded. This solution was kept at 25° C. while continuously stirring.

The acidic reducing and particle modifier solution was prepared byadding 5 g of nitric acid (70% w/w), 5 g maleic acid, 45 g ascorbic acidand 1 g of gold colloid solution to 750 g of deionized water in aseparate container from the acidic aqueous silver salt solution. The pHwas then adjusted to 4 by adding NaOH. This solution was kept at 25° C.while continuously stirring.

The acidic aqueous silver salt solution was slowly added to the acidicreducing and particle modifier solution over a period of one hour toform a reaction mixture that was intensely stirred during the addition.The reaction mixture was maintained at 25° C. Silver particlesprecipitated and were contained within the final aqueous solution. Thefinal aqueous solution was then heated to 70° C. for one hour andstirring continued during the heating.

The final aqueous solution was filtered and the silver powder collected.The silver powder was washed with deionized water until a conductivityof the wash water was less than or equal to 100 microSiemans. The silverpowder was dried for 24 hours at 35° C. in a freeze dryer.

As shown in the scanning electron microscope image of FIG. 1, the silverpowder was comprised of spherically-shaped silver particles. The size Lof the crystallites in the particles was 27.1 nm. d₁₀, d₅₀, and d₉₀ were0.6 mm, 1.6 mm and 3.7 mm, respectively. The silver particles were 99.6%solids. The TMA=−18.

Example 2

The acidic aqueous silver salt solution was prepared by adding 10 g ofgum arabic to 125 g of deionized water. This solution was kept at 25° C.while continuously stirring for an hour. 80 g of silver nitrate was thenadded. This solution was kept at 25° C. while continuously stirring.

The acidic reducing and particle modifier solution was prepared byadding 14 g of gum arabic to 375 g of deionized water in a separatecontainer from the silver salt solution. This solution was kept at 25°C. while continuously stirring for an hour. 3 g of nitric acid (70%w/w), 5 g maleic acid, 45 g ascorbic acid and 1 g of gold colloidsolution was then added to the reducing and particle modifier solution.The pH was then adjusted to 4 by adding NaOH. This solution was kept at25° C. while continuously stirring.

The acidic aqueous silver salt solution was slowly added to the acidicreducing and particle modifier solution over a period of one hour toform a reaction mixture that was intensely stirred during the addition.The reaction mixture was maintained at 25° C. Silver particlesprecipitated and were contained within the final aqueous solution. Thefinal aqueous solution was then heated to 70° C. for one hour andstirring continued during the heating.

The final aqueous solution was filtered and the silver powder collected.The silver powder was washed with deionized water until a conductivityof the wash water was less than or equal to 100 microSiemans. The silverpowder was dried for 24 hours at 35° C. in a freeze dryer.

The size L of the crystallites in the particles was 30.2 nm. d₁₀, d₅₀,and d₉₀ were 0.5 mm, 1.0 mm and 2.0 mm, respectively. The silverparticles were 99.6% solids. The TMA=−17.

Example 3

The acidic aqueous silver salt solution was prepared by adding 10 g ofgum arabic to 125 g of deionized water. This solution was kept at 25° C.while continuously stirring for an hour. 80 g of silver nitrate was thenadded. This solution was kept at 25° C. while continuously stirring.

The acidic reducing and particle modifier solution was prepared byadding 14 g of gum arabic to 375 g of deionized water in a separatecontainer from the silver salt solution. This solution was kept at 25°C. while continuously stirring for an hour. 3 g of nitric acid (70%w/w), 1 g maleic acid, 45 g ascorbic acid and 1 g of gold colloidsolution was then added to the reducing and particle modifier solution.The pH was then adjusted to 4 by adding NaOH. This solution was kept at25° C. while continuously stirring.

The acidic aqueous silver salt solution was slowly added to the acidicreducing and particle modifier solution over a period of one hour toform a reaction mixture that was intensely stirred during the addition.The reaction mixture was maintained at 25° C. Silver particlesprecipitated and were contained within the final aqueous solution. Thefinal aqueous solution was then heated to 70° C. for one hour andstirring continued during the heating.

The final aqueous solution was filtered and the silver powder collected.The silver powder was washed with deionized water until a conductivityof the wash water was less than or equal to 100 microSiemans. The silverpowder was dried for 24 hours at 35° C. in a freeze dryer.

The size L of the crystallites in the particles was 26.2 nm. d₁₀, d₅₀,and d₉₀ were 0.3 mm, 0.7 mm and 1.8 mm, respectively. The silverparticles were 99.6% solids. The TMA=−17.

Comparative Experiment

An acidic aqueous silver salt solution was prepared by adding 14 g ofgum arabic to 250 g of deionized water. This solution was kept at 25° C.while continuously stirring for an hour. Then 80 g of silver nitrate wasadded. This solution was kept at 25° C. while continuously stirring.

An acidic reducing and particle modifier solution was prepared by adding5 g of nitric acid (70% w/w), 5 g maleic acid, 45 g ascorbic acid and 1g of gold colloid solution to 750 g of deionized water in a separatecontainer from the silver nitrate solution. This solution was kept at25° C. while continuously stirring. The pH was not adjusted to between 3and 5.

The acidic aqueous silver salt solution was slowly added to the acidicreducing and particle modifier solution over a period of one hour toform a reaction mixture that was intensely stirred during the addition.The reaction mixture was maintained at 25° C. Silver particlesprecipitated and were contained within the final aqueous solution. Thefinal aqueous solution was then heated to 70° C. for one hour andstirring continued during the heating.

The silver particles were recovered as described in Example 1.

As shown in the scanning electron microscope image of FIG. 2, the silverpowder was comprised of very irregular-shaped relatively large silverparticles. The size L of the crystallites in the particles was 47.6 nm.d₁₀, d₅₀, and d₉₀ were 5.4 μm, 8.7 μm and 14.0 μm, respectively. TheTMA=−7.5%.

What is claimed is:
 1. Silver powder particles made by a processcomprising: (a) preparing an acidic aqueous silver salt solutioncomprising a water soluble silver salt dissolved in deionized water; (b)preparing an acidic aqueous reducing and particle modifier solutioncomprising: (i) a reducing agent selected from the group consisting ofascorbic acid, ascorbates and mixtures thereof; (ii) nitric acid; (iii)maleic acid; and (iv) deionized water, wherein the pH of said acidicaqueous reducing and particle modifier solution is adjusted by theaddition of a base to between 2.5 and 6; (c) maintaining said acidicaqueous silver salt solution and said acidic reducing and surfacemorphology modifier solution at the same temperature, wherein saidtemperature is in the range of 10° C. to 65° C., while stirring eachsaid solution during and after preparation; (d) adding said acidicaqueous silver salt solution to said acidic aqueous reducing andparticle modifier solution while stirring to make a reaction mixture andmaintaining the temperature of said reaction mixture at said temperatureof (c), wherein said silver powder particles precipitate and arecontained within the resulting final aqueous solution; and (e)increasing the temperature of said final aqueous solution to atemperature in the range of 65° C. to 80° C. while continuing stirring;one or both of said acidic aqueous silver salt solution and said acidicaqueous reducing and particle modifier solution further comprising gumarabic and said silver powder particles comprising crystallites of sizeless than or equal to 32 nm as determined by X-ray diffraction and theScherrer formula; and said silver powder particles having a d₅₀ in therange of 0.5 to 3.5 μm.
 2. The silver powder particles of claim 1, saidprocess further comprising: (f) separating said silver powder particlesfrom said final aqueous solution; (g) washing said silver powderparticles with deionized water; and (h) drying said silver powderparticles.
 3. The silver powder particles of claim 1, wherein in saidprocess said base used to adjust the pH of said acidic aqueous reducingand particle modifier solution is NaOH.
 4. The silver powder particlesof claim 1, wherein in said process said water soluble silver salt issilver nitrate and said reducing agent is ascorbic acid.
 5. The silverpowder particles of claim 1, wherein in said process said pH of saidacidic aqueous reducing and particle modifier solution is adjusted tobetween 3 and
 5. 6. The silver powder particles of claim 1, wherein insaid process said acidic aqueous reducing and particle modifier solutionfurther comprising a metal colloid selected from the group consisting ofgold colloid and silver colloid.
 7. The silver powder particles of claim1, wherein in said process said temperature in step (c) is in the rangeof 10° C. to 35° C. and the temperature in step (e) is in the range of65° C. to 75° C.
 8. A silver thick film paste comprising silver powderparticles made by a process comprising: (a) preparing an acidic aqueoussilver salt solution comprising a water soluble silver salt dissolved indeionized water; (b) preparing an acidic aqueous reducing and particlemodifier solution comprising: (i) a reducing agent selected from thegroup consisting of ascorbic acid, ascorbates and mixtures thereof; (ii)nitric acid; (iii) maleic acid; and (iv) deionized water, wherein the pHof said acidic aqueous reducing and particle modifier solution isadjusted by the addition of a base to between 2.5 and 6; (c) maintainingsaid acidic aqueous silver salt solution and said acidic reducing andsurface morphology modifier solution at the same temperature, whereinsaid temperature is in the range of 10° C. to 65° C., while stirringeach said solution during and after preparation; (d) adding said acidicaqueous silver salt solution to said acidic aqueous reducing andparticle modifier solution while stirring to make a reaction mixture andmaintaining the temperature of said reaction mixture at said temperatureof (c), wherein said silver powder particles precipitate and arecontained within the resulting final aqueous solution; and (e)increasing the temperature of said final aqueous solution to atemperature in the range of 65° C. to 80° C. while continuing stirring;one or both of said acidic aqueous silver salt solution and said acidicaqueous reducing and particle modifier solution further comprising gumarabic and said silver powder particles comprising crystallites of sizeless than or equal to 32 nm as determined by X-ray diffraction and theScherrer formula; and said silver powder particles having a d₅₀ in therange of 0.5 to 3.5 μm.
 9. The silver thick film paste of claim 8, saidprocess further comprising: (f) separating said silver powder particlesfrom said final aqueous solution; (g) washing said silver powderparticles with deionized water; and (h) drying said silver powderparticles.
 10. The silver thick film paste of claim 8, wherein in saidprocess said base used to adjust the pH of said acidic aqueous reducingand particle modifier solution is NaOH.
 11. The silver thick film pasteof claim 8, wherein in said process said water soluble silver salt issilver nitrate and said reducing agent is ascorbic acid.
 12. The silverthick film paste of claim 8, wherein in said process said pH of saidacidic aqueous reducing and particle modifier solution is adjusted tobetween 3 and
 5. 13. The silver thick film paste of claim 8, wherein insaid process said acidic aqueous reducing and particle modifier solutionfurther comprising a metal colloid selected from the group consisting ofgold colloid and silver colloid.
 14. The silver thick film paste ofclaim 8, wherein in said process said temperature in step (c) is in therange of 10° C. to 35° C. and the temperature in step (e) is in therange of 65° C. to 75° C.
 15. A semiconductor device comprising anelectrode formed by firing a silver thick film paste containing silverparticles, said silver powder particles made by a process comprising:(a) preparing an acidic aqueous silver salt solution comprising a watersoluble silver salt dissolved in deionized water; (b) preparing anacidic aqueous reducing and particle modifier solution comprising: (i) areducing agent selected from the group consisting of ascorbic acid,ascorbates and mixtures thereof; (ii) nitric acid; (iii) maleic acid;and (iv) deionized water, wherein the pH of said acidic aqueous reducingand particle modifier solution is adjusted by the addition of a base tobetween 2.5 and 6; (c) maintaining said acidic aqueous silver saltsolution and said acidic reducing and surface morphology modifiersolution at the same temperature, wherein said temperature is in therange of 10° C. to 65° C., while stirring each said solution during andafter preparation; (d) adding said acidic aqueous silver salt solutionto said acidic aqueous reducing and particle modifier solution whilestirring to make a reaction mixture and maintaining the temperature ofsaid reaction mixture at said temperature of (c), wherein said silverpowder particles precipitate and are contained within the resultingfinal aqueous solution; and (e) increasing the temperature of said finalaqueous solution to a temperature in the range of 65° C. to 80° C. whilecontinuing stirring; one or both of said acidic aqueous silver saltsolution and said acidic aqueous reducing and particle modifier solutionfurther comprising gum arabic and said silver powder particlescomprising crystallites of size less than or equal to 32 nm asdetermined by X-ray diffraction and the Scherrer formula; and saidsilver powder particles having a d₅₀ in the range of 0.5 to 3.5 μm. 16.The semiconductor device of claim 15, said process further comprising:(f) separating said silver powder particles from said final aqueoussolution; (g) washing said silver powder particles with deionized water;and (h) drying said silver powder particles.
 17. The semiconductordevice of claim 15, wherein in said process said base used to adjust thepH of said acidic aqueous reducing and particle modifier solution isNaOH.
 18. The semiconductor device of claim 15, wherein in said processsaid water soluble silver salt is silver nitrate and said reducing agentis ascorbic acid.
 19. The semiconductor device of claim 15, wherein insaid process said pH of said acidic aqueous reducing and particlemodifier solution is adjusted to between 3 and
 5. 20. The semiconductordevice of claim 15, wherein in said process said acidic aqueous reducingand particle modifier solution further comprising a metal colloidselected from the group consisting of gold colloid and silver colloid.21. The semiconductor device of claim 15, wherein in said process saidtemperature in step (c) is in the range of 10° C. to 35° C. and thetemperature in step (e) is in the range of 65° C. to 75° C.